I am not sure how your sharepoint backend is expecting parameters to be. From the error it looks like parameters are invalid. Can you please try verifying the WSDL operation's expected parameters?
I am trying to connect to my SharePoint document library. I know that SharePoint document library can be accessed via a _vti_bin/Lists.asmx?wsdl. I do not have control over the SharePoint Root directory so I cannot set up the crossdomain.xml policy. What I would like to know is, would it be possible for me to pass login credentials to SharePoint to query the lists.asmx webservcie?
Do you have a good example that you could provide?
It sounds like you and I have a similar situations.
I have had success querying SharePoint lists using dspsts.asmx webservice. There is a great write up here:
My method now for getting data/updating data is:
dspsts.asmx - query the list (this can be as specific a query as you desire, for the most part)
Lists.asmx - update the list (using UpdateListItems method)
As long as you host your Flex app on the site you are trying to access, you don't need a crossdomain.xml policy. If that's not your situation, then I'm not not sure how you would next proceed.